1. Field of the Invention
The present invention relates to a high density magnetic recording technology and particularly to a patterned media capable of high density perpendicular magnetic recording and a method for manufacture thereof.
2. Description of the Related Art
In recent years, with the progress of multimedia application of data such as pictures, images, and voice, and an amount of information data for searching per user has increased. Therefore, a larger capacity and higher speed of database are required. Meanwhile, due to improvement in the surface recording densities of magnetic recording media associated with increases in the recording capacity of Hard Disk Drives (HDDs), each record bit size of a magnetic recording medium is becoming extremely fine to be about several tens nm. In order to obtain a reproducing output from this very fine recording bit, as large a saturation magnetization and film thickness as possible need to be secured for each bit. However, when the recording bit is made fine, the volume of switching unit (V) of each bit is reduced, involving a problem such that magnetization inversion due to thermal fluctuation causes loss of magnetized information.
Generally, the influence of this thermal fluctuation becomes larger as the value of Ku·V/kT becomes smaller, where ku is an anisotropy constant, V is a volume of switching unit, k is the Boltzmann's constant, and T is the absolute temperature. It is experientially said that when the Ku·V/kT is less than 100, magnetization reversal due to thermal fluctuation occurs. Magnetic anisotropic energy, which is required to keep the magnetization orientation of a magnetic particle to be in one direction, is expressed as the product of magnetic anisotropic energy density Ku and the volume V of the magnetic particle. If a value of the Magnetic anisotropic energy is as large as a value of the thermal fluctuation energy, the magnetization fluctuates with time and the phenomenon occurs that recorded information is lost.
In a longitudinal magnetic recording type of magnetic recording medium, since the demagnetized field in recording bits in high recording density areas is strong, it is more likely to be affected by the thermal fluctuation even if the magnetic particle size is relatively large. On the other hand, in a perpendicular magnetic recording type of magnetic recording medium, by growing magnetic particles in the film thickness direction, a volume of switching unit V can be made large as the particle size on the medium surface becomes smaller, and thus the influence of thermal fluctuation can be suppressed. However, as high density of a magnetic medium is further promoted from now on, the thermal fluctuation resistance becomes limited even with the perpendicular magnetic recording type.
As a medium for solving the problem of the thermal fluctuation resistance, a magnetic recording medium called a “patterned media” is drawing attention. The patterned media usually means a magnetic recording medium having a plurality of magnetic areas, which are to become recording bit units, and which are respectively formed independently from each other in a non-magnetic-material layer. In other words, the patterned media can be said to be a medium having a magnetically-continuous magnetic thin film divided into the size of the recording magnetic domain. In a usual patterned media, oxide such as SiO2, Al2O3, or TiO2, nitride such as Si3N4, AlN, or TiN, carbide such as TiC, or a boric compound such as BN is used as the non-magnetic substance layer, and in this non-magnetic-substance layer, ferromagnetic material areas are formed selectively.
Because the patterned media is a magnetic thin film divided into the size of the recording magnetic domain, the magnetization minimum unit volume V can be enlarged, and thus the problem of thermal fluctuation can be avoided. In a conventional continuous magnetic thin film, the number of magnetic particles used is allowed up to about 1000 grains per bit. However, the number of grains corresponding to 1-bit decreases as the recording density becomes higher. Since recording mark edges are determined by the grain boundaries, the grains need to be as small as possible in order to ensure S/N. Accordingly, in the conventional continuous magnetic film, V is forced to be smaller. On the other hand, in the patterned media, the edges of recording magnetic domains can be defined structurally. Therefore, the improvement of S/N can be expected without making the V smaller.
In the patterned media, because the ferromagnetic areas, serving as recording bit units, are respectively formed independently from each other, interference between recording bits can be prevented. This structure works for suppressing the record loss and noise generated due to adjacent bits. Furthermore, domain wall displacement resistance increases by patterning, thereby making it possible to improve the magnetic characteristic.
As described above, because the patterned media can suppress the magnetization inversion due to the thermal fluctuation, it is effective as a high-density magnetic recording medium, but the manufacturing process is more complex than the other magnetic recording media.
FIGS. 1A to 1E show a general manufacturing method of the patterned media used conventionally. According to the conventional producing method, first a ferromagnetic thin film layer 120 including ferromagnetic-materials, such as Fe, Co, Ni, etc., is formed on a substrate 110 (FIG. 1A), and the ferromagnetic thin film layer 120 is etched by ion-milling using a resist pattern 130 as a mask (FIG. 1B), and an independent pattern is formed for each recording bit (FIG. 1C). Further, the surface is coated with a non-magnetic layer 140 (FIG. 1D), and finally the surface is polished so as to expose a ferromagnetic pattern (FIG. 1E).
Note that as shown in FIG. 1B, because the ferromagnetic thin film layer 120 is made of a material to which etching is difficult to apply. Therefore, chemical etching using Reactive Ion Etching (RIE), etc., which is widely used in semiconductor processes, is difficult to be used, and thus, physical etching such as ion beam-milling is used instead.
However, because ions accelerated by an electric field are sputtered onto the sample surface, the ion beam-milling damages the processed surface. This damage may cause noise during reproducing and recording. Therefore, in order to improve the magnetic characteristic, the development of a manufacturing method causing no damage is desired. Furthermore, there is a problem that manufacturing costs are great due to many process steps, and thus the development of a simpler manufacturing method is desired.
On the other hand, a single magnetic pole head is employed as a writing/reading head suitable for the perpendicular magnetic recording type of magnetic recording medium. Also in the case of the perpendicular magnetic recording type of patterned media, this single magnetic pole head is preferably used during writing and reading. Although it is possible to write into very small areas with the single magnetic pole head having the magnetic pole made smaller to converge leak magnetic fields, a magnetic loop from the head to the medium and back to the head needs to be formed and magnetic flux needs to be guided efficiently through the coils of the head. Therefore, when using the single magnetic pole head, a soft magnetic layer, which is to be a path for the magnetic flux, is preferably arranged, as the base of a magnetic recording layer in order to form the magnetic loop.
Therefore, when considering the structure and a method of manufacturing of the perpendicular magnetic recording type of patterned media, it is preferable to have a structure where a soft magnetic layer which is to be a path for the magnetic flux is arranged between a recording layer and a non-magnetic substrate, requiring a method for manufacturing such a structure. However, if the domain walls occur in the soft magnetic layer, they will cause noise during writing and reading.